1. Field of the Invention
This invention relates to catalytic materials for the treatment of gas streams, and in particular to catalytic materials effective for the reduction of nitrogen oxides (NO.sub.X). Nitrogen oxides are well-known noxious by-products of the high temperature combustion of hydrocarbon fuels by internal combustion engines. The release of these oxides as exhaust emissions has caused environmental concerns leading to governmental regulations that compel motor vehicle manufacturers to abate the emission of NO.sub.X. Efforts towards compliance with such regulations are complicated by the fact that the exhaust gas streams containing NO.sub.X typically also contain other pollutants which are typically abated through oxidation reactions, whereas NO.sub.X abatement is typically carried out as a reduction reaction in which it is desired to reduce the NO.sub.X to nitrogen while minimizing the formation of N.sub.2 O and sulfates. Conventional three-way catalysts, which are known for their ability to abate hydrocarbons, carbon monoxide and NO.sub.X under stoichiometric conditions, are not effective for NO.sub.X reduction in lean environments, i.e., in environments in which oxygen is present in excess of the stoichiometric quantity required to oxidize the hydrocarbons, partially burned hydrocarbons and carbon monoxide in the gas stream.
2. Related Art
One known method for the reduction of NO.sub.X from lean emissions is to flow the exhaust gas containing the NO.sub.X in contact with a zeolite catalytic material comprising, for example, ZSM-5, which has been ion-exchanged with copper. Such catalyst was found to reduce NO.sub.X under lean conditions using unburned hydrocarbons in the exhaust gas as reductants, and was found to be effective at temperatures from about 350.degree. C. to 550.degree. C. However, such catalysts are often lacking in durability, in that catalytic performance usually decreases significantly after exposure of the catalyst to high temperature steam and/or SO.sub.2.
Catalysts based on platinum-containing materials have also been found to abate NO.sub.X in lean environments, but such catalysts tend to produce excessive quantities of N.sub.2 O, and also to oxidize SO.sub.2, which is present in the exhaust as a result of the oxidation of the sulfur component of fuels, to SO.sub.3 . Both products are undesirable; N.sub.2 O fosters an environmental greenhouse effect while SO.sub.3 contributes to the formation of particulate matter in exhaust emissions by reacting to form sulfates which add to the particulate mass. Accordingly, there is a need for a catalyst that reduces NO.sub.X to N.sub.2 while producing only limited quantities of N.sub.2 O and SO.sub.3.
Japanese Patent H1-135541 (1989) of Toyota Jidosha K.K. et al discloses a catalyst for reducing NO.sub.X in lean car exhaust comprising zeolites that contain one or more platinum group metals, including ruthenium, by ion-exchange into the zeolite. In the exemplified embodiments, 100 grams of a washcoat comprising 150 parts zeolite and 40 parts of a mixture of alumina sol and silica sol having a 50:50 Al:Si ratio is coated onto a carrier. The following amounts of platinum group metals are then incorporated into the zeolites: in Examples 1 and 2, 1.0 gram platinum (1.27% by weight of zeolite plus platinum) and 0.2 grams rhodium (0.25% by weight zeolite plus rhodium); Example 3, 1.0 gram palladium; Example 4, 1.2 grams ruthenium (1.5% by weight zeolite plus ruthenium); Example 5, 1.2 grams iridium. Comparative examples were prepared without zeolite.
U.S. Pat. No. 5,330,732 to Ishibashi et al, dated Jul. 19, 1994, teaches that one or more of platinum, palladium and rhodium can be loaded onto zeolites "by an ion exchange and by an immersion" (column 3, lines 11-17 and 22-30) to produce NO.sub.X -reducing catalysts. Durability is improved by using at least 1.3 parts platinum. The platinum group metals are used separately in the following amounts per 100 parts by weight ("parts") of zeolite; platinum, 1.3 parts or more; palladium, 0.8 parts or more; or rhodium, 0.7 parts or more. In terms of the weight of the metals as a percent of the combined weight of the metal plus zeolite, these quantities correspond to 1.28% platinum, 0.79% palladium, and 0.7% rhodium. The graphs of FIGS. 1-6 of Ishibashi et al plot NO.sub.X conversion against platinum group metal loadings and show data points which appear to start at about 0.2 parts of platinum group metal, about 0.2%. However, the data show that the claimed amount of at least about 1.28% of platinum must be used to attain satisfactory NO.sub.X conversion. Preferred zeolites have a pore size of 5 to 10 Angstroms.
U.S. Pat. No. 4,206,087 to Keith et al, dated Jun. 3, 1980, teaches that a NO.sub.X -reducing catalyst may comprise 0.01 to 4 weight percent, preferably 0.03 to 1 weight percent platinum group metals dispersed on an inorganic support material that may comprise an alumino-silicate.
U.S. Pat. No. 5,041,272 to Tamura et al, dated Aug. 20, 1991, teaches that hydrogen-form zeolites are catalytically effective NO.sub.X -reducing catalyst materials at 400.degree. C. (see Example 1, column 3).